1. Streptococcus pneumoniae 
Streptococcus pneumoniae is a Gram-positive bacterium with capsular polysaccharide. As an ordinary pathogenic bacterium, it is the leading pathogenic agent of community-acquired pneumonia, and can also result in other diseases such as sinusitis, otitis media and meningitis. In developing countries, pneumonia caused by Streptococcus pneumoniae is one of the major causes of death in elderly people and young children: and in developed countries, otitis media caused by Streptococcus pneumoniae is also one of the most common diseases in the pediatric population. It was estimated that approximately one million children under age 5 worldwide died from diseases caused by Streptococcus pneumoniae in the year 2000. Christa L Fischer Walker et al. estimated that about fourteen million children under age 5 worldwide developed into severe pneumonia in the year 2010. Streptococcus pneumoniae is a leading pathogen of severe pneumonia; and throughout the world, at least 18% of severe cases and 33% of death cases caused by pneumonia are resulted from Streptococcus pneumonia. 
2. Research Status of Current Pneumococcal Vaccines
At present, commercially available pneumococcal vaccines mainly include polysaccharide vaccines and polysaccharide-conjugate vaccines, both of which are pneumococcal capsular polysaccharide type-specific vaccines. Based on the difference of pneumococcal capsular polysaccharides, more than 90 different serotypes have been identified. Thus, pneumococcal vaccines based on polysaccharide cannot cover all the pneumococcal serotypes. The 23-valent pneumococcal capsular polysaccharide vaccine is ineffective to children under age 2 whose immune system has not been fully developed, and the 7-valent pneumococcal conjugate vaccine that has been widely used can only encompass 7 serotypes which account for 55% of pathogenic serotypes in Asia. Moreover, with the applications of polysaccharide vaccines and polysaccharide-conjugate vaccines, a phenomenon of serotype replacement has emerged, resulting in decreasing efficacy of vaccines year by year. Also, polysaccharide-conjugate vaccines are hardly available in developing countries owing to their high costs. Although the World Health Organization (WHO) recommended in 2007 that pneumococcal conjugate vaccines should be included in National Immunization Program in developing countries, this has not been materialized to date.
Pneumococcal protein vaccines have become the hot topic of current researches on pneumococcal vaccines because of their characteristics such as serotype independency, relatively lower costs, and good immunogenicity.
3. Pneumococcal Surface Adhesion A
Pneumococcal surface adhesion A (hereinafter referred to as “PsaA”) is a highly-conserved, species-specific, surface-bound lipoprotein expressed by all strains of Streptococcus pneumoniae, and having a molecular weight of 37 KD and good immunogenicity. PsaA, which plays a vital role during manganese delivery and adherence of Streptococcus pneumoniae to mucosal membrane of respiratory tract, is an important virulence factor of Streptococcus pneumoniae invasion. It was reported that PsaA mutant affects many essential functions of Streptococcus pneumoniae including adhesion and virulence. PsaA exhibits excellent immunogenicity. Literatures reported that high-titre PsaA antibody can be induced either after PsaA immunization or after colonization of Streptococcus pneumoniae in nasopharynx, and also evaluated immune protective efficacy of PsaA. Therefore, PsaA protein, characterized by its species-specificity, sequence-conservatism, higher immunogenicity and immune protection, has become one of the hot topics in researches on Streptococcus pneumoniae protein vaccines.
4. Pneumococcal Surface Protein A
Pneumococcal surface protein A (hereinafter referred to as “PspA”) is a bacterial surface protein belonging to the choline-binding protein family, and an important virulence factor of Streptococcus pneumoniae. PspA serves main functions of binding to lactoferrin to result in loss of its bacteriostatic activity and inhibiting complement deposition on the surface of Streptococcus pneumoniae so as to interfere in complement-mediated opsonization and phagocytosis. Studies have revealed that PspA− strain has lower virulence and is immediately cleared by body after infection. Actively-immunizing recombinant PspA protein can induce protection against various Streptococcus pneumoniae challenge models.
Currently, PspA was found to be present in all clinically isolated strains of Streptococcus pneumoniae. Nevertheless, such PspAs are not conservative in their molecular structures and have molecular weights varied from 67 to 100 KD. PspA molecule includes five regions: a signal peptide region, an α-helical region, a proline-rich region, a choline-binding region in which PspA binds to a cell surface, and a C-terminal tail of 17 amino acids.
Despite the variability of PspA in its structure and antigenicity, antibodies generated against PspA are highly cross-reactive and cross-protective. As shown by researches on gene and protein mapping, the majority of cross protection-conferring antigen epitopes are located in a sequence of approximately 100 amino acids adjacent to the proline-rich region in the α-helix region, which is called as a Clade Defining Region (hereinafter referred to as “CDR”). Based on the variations within the CDR regions, PspA is classified into three families (Fam1, Fam2 and Fam3), which are subdivided into 6 clades (Clade1, Clade2, Clade3, Clade4, Clade5 and Clade6), wherein Clade1 and Clade2 belong to Fam1; Clade3, Clade4 and Clade5 belong to Fam2, and Clade6 belongs to Fam3. Gene sequences of CDR regions in the clades of the same family share more than 80% identity; and gene sequences of CDR regions in different families share more than 50% identity. In the three families, Fam1 and Fam2 have prevalence of greater than 98%.
Most protein vaccines in the prior art utilize a single protein and can only act against certain category of Streptococcus pneumoniae, and thus do not have a broad spectrum. The present invention increases broad spectrum and protection of vaccines by using a fusion protein of PsaA and PspA, and using said fusion protein in combination with further protein antigens or as a protein carrier.